A video output driver stage is used in video signal processing and display systems such as television receivers or video monitors to supply high level video signals to an intensity control electrode of an image reproducing kinescope. A cascode amplifier configuration exhibits a good high frequency response among other features, and is therefore advantageously used as a video output driver amplifier. A cascode video output amplifier employing bipolar transistors, for example, typically includes a low voltage input pre-driver transistor arranged as a common emitter amplifier, and a high voltage output driver transistor arranged as a common base amplifier. The collector output of the common emitter input transistor drives the emitter input of the common base output transistor. In the case of a color television receiver, a cascode video driver stage is associated with each of the red, green and blue color signal channels.
In many television receiver designs it is desirable to situate the pre-driver transistor on a circuit board containing low level video signal processing circuits, and to situate the higher power output driver transistor on a circuit board attached to the neck of the kinescope in the vicinity of the kinescope socket. In this way parasitic capacitances presented to the output circuit of the driver transistor are greatly reduced because the collector output of the driver transistor is located close to the driver cathode intensity control electrode of the kinescope and, as a result, parasitic capacitances associated with the wire connecting the output of the driver transistor and the kinescope cathode are minimized. This arrangement is substantially immune to the effects of parasitic capacitance and stray signal pick-up associated with a relatively long wire (e.g., approximately 8 to 10 inches in length) connecting the collector output of the pre-driver transistor to the emitter input of the driver transistor. This results because of the low impedance at the emitter input of the driver transistor, and because of the relatively strong signal being conveyed by the connecting wire.
However, the connecting wire can undesirably act as an antenna for radiating high frequency components of the video signal and harmonics thereof, which can be picked-up by sensitive circuits of the receiver such as RF tuner circuits and intermediate frequency (IF) signal processing circuits. The radiated high frequency components represent interference signals which can adversely affect the operation of the RF and IF circuits such that distortion of a reproduced video image results.
The magnitude of the radiated high frequency interference signals can be reduced by reducing the level of high frequency currents flowing in the connecting wire. One way to accomplish this is by means of a low pass filter coupled prior to the pre-driver amplifier to attenuate the high frequency components of the video signal. The desired high frequency response can be restored afterwards by means of a peaking network associated with the output circuit of the driver transistor. Such a peaking network commonly includes a peaking coil in the collector output circuit of the driver transistor. However, the ability of peaking coil to fully restore the attenuated high frequency components of the video signal is limited by several factors related to the design of the driver stage, so that the peaking coil may be unable to restore the attenuated high frequency response.